Acetic acid: Microwave spectra, internal rotation and substitution structure

Abstract

The internal rotation splittings in the microwave spectrum of acetic acid have been re-examined, using both principal axis method (PAM) and internal axis method (IAM) treatments. It is shown how individual terms in the PAM equation can be correlated to the first terms in an expansion of the corresponding IAM formula. When centrifugal distortion was allowed for, both methods reproduced the A-type frequencies within experimental error. For the E-type lines the r.m.s. deviation was 1.64 MHz in the PAM (using W vσ ( n) -terms with n = 1, 2, …, 14, d, e, f) and 0.70 MHz in the IAM. To derive the substitution structure eight isotopic species were studied. The inertial moments of the molecules with various degrees of methyl group deuteration are not consistent with each other, so these data could not be fully used. Therefore, the structure was derived with the assumption of a cylindrically symmetric methyl group, although there is some evidence that the HCH angles differ by a few degrees. The geometry, which is in agreement with earlier electron diffraction results, is compared with the substitution structures of other carboxylic acids.